Works great. It even trips the little lightup flash indicator in the viewfinder of the FM2!
Be careful the contacts are clean and making a good connection though - some of those FM2's have hotshoes that are quite old.
I'd also suggest using it in "A" mode as it works great along with the FM2 since it is all manual. M mode of course works just fine, but you do have to set the flash power correctly.
You should also be able to use the PC sync on the FM2 to trip almost any flash (SB800 included)